Digital Subscriber Line (DSL) technology using copper loops, e.g., including ADSL, ADSL2, (S)HDSL, VDSL, VDSL2, and the upcoming G.fast, during all its history, attempted to increase the bit rate in the aim to deliver more broadband services to the customer. Since copper loops deployed from a Central Office (CO) to a customer premises equipment (CPE) are typically rather long and do not allow transmission of data with bit rates more than few Mb/s, modern access networks use street cabinets, MDU-cabinets, and similar arrangements, also referred to as distribution points (DP): the cabinet or other DP is connected to the CO by a high-speed fiber communication line, e.g., gigabit passive optical network (GPON) and installed close to the customer premises. From these cabinets, high-speed DSL systems, such as Very-High-Bit-Rate DSL (VDSL), provide connection to the CPE. The currently deployed VDSL systems (ITU-T Recommendation G.993.2) have range of about 1 km, providing bit rates in the range of tens of Mb/s. To increase the bit rate of VDSL systems deployed from the cabinet, the recent ITU-T Recommendation G.993.5 defined vectored transmission that allows increasing upstream and downstream bit rates up to 100 Mb/s. Vectoring will also be used in upcoming G.fast.
Recent trends in the access communications market show that data rates up to 100 Mb/s which are provided by VDSL systems using Vectoring as defined in ITU-T Recommendation G.993.5 are not sufficient and bit rates up to 1.0 Gb/s are required.
To achieve these targets, copper loops connecting the CPE must be as short as 50-100 m. Operation using such short loops requires installation of many small DPs, e.g, in the form of street/MDU cabinets, that serve a very small number of customers, e.g., 16 or 24, and are connected to the backbone via fiber. Such access scenarios are also referred to as FTTdp (fiber to the distribution point).
As for example described by G. Ginis and J. M. Cioffi, “Vectored transmission for digital subscriber line systems”, Selected Areas in Communications, IEEE Journal on, 20(5), pp. 1085-1104 (2002), vectoring may be used in systems operating from a DP, to reduce far-end crosstalk (FEXT) and obtain high bit rates. To improve energy efficiency and to and reduce hardware complexity, synchronized time division duplexing (STDD) may be used for FTTdp instead of frequency division duplexing (FDD) as typically used in VDSL.
The DPs shall allow very flexible installation practices: For example, they should be light and easy to install on a pole or house wall, or in a basement, without air-conditioning. The most challenging issue for these flexible connection plans is providing DPs with power. Here, one solution is so-called “reverse feeding” where the equipment of the DP is fed by the connected customer. The requirement of reverse power feeding and the small size of the DP may imply substantial restrictions on the power consumption of the DP.
The main contributors to the power consumption of the DP are DSL transceivers. Further, in many cases also battery-powered operation of the CPE is required, e.g., to support life line POTS (Plain Old Telephone System) during power outages. The latter implies low power requirements also for DSL transceivers of the CPE equipment.
To support power efficient operation, a method of power saving called “discontinuous operation” may be used.
Conventional DSL systems transmit data continuously on all lines sharing the cable binder. Whenever there is no data available, idle bytes are transmitted. With this type of static operation, the system stability and performance is maintained. Link quality measures like signal-to-noise-ratio (SNR) or bit error rate (BER) change only slowly over time. Therefore, they can be measured by averaging over a long time period.
In FTTdp applications, the transmit power is only a small portion of the overall power consumption, because the aggregate transmit power is in the range of 4 dBm. Components like the analog front-end (AFE) and digital front-end (DFE) electronics consume power irrespective of the transmit power, and may significantly contribute to the overall power consumption, because they operate at much higher frequencies of typically 100 MHz or 200 MHz in comparison to 8 MHz-30 MHz in VDSL.
In order to provide significant power savings in discontinuous operation, also analog and digital components of the transceiver, such as AFE and DFE may be switched into low-power (standby) state, as for example described by L. Humphrey and I. Horsley, “G.fast: Discontinuous Operation”, Contribution ITU-T SG15/Q4a 11BM-035 (2011).
In currently operated systems using Vectoring, such as specified in ITU-T Rec. G.993.5-2010. Self-FEXT CANCELLATION(Vectoring) for Use with VDSL2 Transceivers (2010), a time consuming procedure called “orderly leaving” is required before a link can be switched off. If a line is disconnected without orderly leaving, the remaining active lines of the binder may experience substantial performance drops. Therefore, AFE and DFE cannot be turned off for short time which substantially reduces possible power savings by discontinuous operation.
Due to the fast changes of the link quality which are caused by discontinuous operation, the SNR measurement as it is implemented in conventional DSL technology may deliver incorrect results and lead to a wrong estimation of the link rate. When the data rates are over-estimated, this may cause bit errors during transmission. When the data rates are under-estimated, the actual data rate is unnecessarily reduced.
Accordingly, there is a need for technologies which allow for efficient performance monitoring, e.g., for link quality monitoring for low power modes, such as low power modes involving discontinuous operation of lines of a data transmission system.